Learn Hardcore French: Le médecin lui dit que la méditation et le tennis sont bons pour son dos. - The doctor tells her that meditation and tennis are good for her back.

Here lui is an indirect object pronoun meaning to him / to her.
You cannot say dit lui in standard French; the correct order is lui dit.

What exactly does lui mean here? Is it “him” or “her”? Why is the same word used?

Lui is the indirect object pronoun for both:

to him
to her

So in English you’d choose him or her depending on context:

Le médecin lui dit... = The doctor tells him / her...

Only context (previous sentences, story, etc.) lets you know the person’s gender. French doesn’t distinguish here; lui covers both.

Why is the verb dit in the present tense instead of a past tense like a dit?

Both are possible, but they’re used differently:

Le médecin lui dit que...
→ present tense (dit) = The doctor tells him/her that... (either now, or in a narrative present)
Le médecin lui a dit que...
→ past tense (a dit) = The doctor told him/her that...

So the sentence as written is talking about something presented as current (or told in a “live” narrative style).

Why is it la méditation but le tennis? How do I know the genders?

Noun gender in French is mostly arbitrary and must be learned:

la méditation – feminine
le tennis – masculine

There isn’t a strong logical reason here; it’s simply how the language has evolved. A dictionary will always list nouns with le / la (or m. / f.) so you can memorize the gender along with the word.

Why is it sont bons and not est bon?

The subject is la méditation et le tennis → that’s two things, so it’s plural.

Singular:
- La méditation est bonne pour le dos.
- Le tennis est bon pour le dos.
Plural together:
- La méditation et le tennis sont bons pour le dos.

So:

sont = 3rd person plural of être
bons = masculine plural form of bon, agreeing with la méditation et le tennis (mixed gender → masculine plural).

Why is the adjective bons masculine plural, even though méditation is feminine?

In French, when you have a mixed-gender group, any adjective referring to the group becomes masculine plural, even if only one element is masculine:

la méditation – feminine
le tennis – masculine

Together: la méditation et le tennis → masculine plural
Therefore:

bons (masc. plural), not bonnes (fem. plural).

Why does it say son dos and not le dos? In English we say “his back” or “her back”, but in some French sentences for body parts I’ve seen le.

French uses both patterns, depending on what you want to emphasize.

Possessive determiner (son, sa, ses)
Used here: son dos = his/her back.
This clearly says whose back is concerned.
Definite article (le, la, les)
Common in expressions with a reflexive pronoun:
- Il s’est cassé le dos. = He broke his back.
  Here le is used because se already shows whose body part is affected.

In your sentence there is no reflexive pronoun, so son dos naturally emphasizes “the patient’s back” (or whoever lui is).

Whose back is son dos referring to? The doctor’s or the person he’s speaking to?

Grammatically, son could refer to any previously mentioned masculine or singular person, but:

The most natural reading is: the back of the person referred to by “lui” (the patient / listener).
Context would make it clear. Without context, a neutral translation is:
- The doctor tells him/her that meditation and tennis are good for his/her back.

If the back were the doctor’s, the sentence would usually be rephrased for clarity.

Why do we use pour here: bons pour son dos? Can we say something else instead of pour?

Pour expresses the idea of benefit or suitability:

bon pour la santé – good for your health
mauvais pour le cœur – bad for the heart

So bons pour son dos = good for his/her back.

Other prepositions wouldn’t work here; you really need pour to show “good for something”.

Why do we say la méditation and le tennis with articles when in English we often say “Meditation and tennis are good for your back” (no “the”)?

French normally uses the definite article when talking about things in a general or generic way:

La musique adoucit les mœurs. – Music soothes the soul.
Le sport est bon pour la santé. – Sport is good for your health.

So:

La méditation et le tennis sont bons pour son dos.
Literally: The meditation and the tennis are good for his/her back, but in French this is the normal way to talk about meditation and tennis in general.

Leaving out the article

∗Méditation et tennis sont bons...
is possible but sounds quite stylistic or headline-like.

Is médecin always masculine? What if the doctor is a woman?

Traditionally:

The noun médecin is grammatically masculine: un médecin, le médecin.
Even if the doctor is a woman, many speakers still say le médecin.

Usage is changing, and some people now say:

une médecin, la médecin for a female doctor.

However, in standard, traditional grammar, médecin is masculine, and you will commonly see le médecin used for both male and female doctors. The pronoun referring to the person (outside this sentence) can be il or elle depending on the doctor’s real gender.

Could we rewrite the sentence differently, for example: La méditation et le tennis lui sont bons pour le dos? Is that correct?

Yes, that sentence is grammatically correct, but the nuance and focus change a bit:

Le médecin lui dit que la méditation et le tennis sont bons pour son dos.
→ Normal, straightforward reporting of what the doctor says.
La méditation et le tennis lui sont bons pour le dos.
→ More unusual word order; it emphasizes la méditation et le tennis as the subject from the start.
It sounds a bit more formal or literary.

Native speakers are more likely to stick with the original structure when simply reporting the doctor’s advice.