يستعمل صديقي كوب القياس من أجل سكب الصلصة.

Arabic often allows a verb-first sentence order. So يستعمل صديقي... is a normal way to say My friend uses...

You could also say:

صديقي يستعمل كوب القياس...

That version is also correct, but it puts a bit more emphasis on my friend. Both are natural.

يستعمل means he uses / he is using.

The يـ at the beginning marks the 3rd person masculine singular in the imperfect verb system, so it means he.

So:

• يستعمل = he uses
• the base verb is استعمل = to use

Depending on context, this can mean:

• he uses
• he is using
• he uses habitually

Because صديقي is made of:

• صديق = friend
• ـي = my

So صديقي literally means friend-my, which in natural English is my friend.

This is a very common Arabic way to show possession:

• كتابي = my book
• بيتي = my house
• صديقي = my friend

This is an iḍāfa construction, often called a genitive/possessive construction.

Literally, كوب القياس means something like:

cup of measuring

In natural English, that becomes:

measuring cup

In an iḍāfa:

• the first noun comes first: كوب
• the second noun specifies it: القياس

A few important things about iḍāfa:

• the first noun usually does not take ال
• the second noun can take ال if the whole phrase is definite

So كوب القياس is the normal Arabic structure for the measuring cup / a measuring cup for measuring, depending on context.

Because in an iḍāfa construction, the first noun usually does not take ال.

So Arabic says:

• كوب القياس

not usually:

• الكوب القياس

The definiteness of the whole phrase is determined by the second part. Since القياس is definite, the whole phrase can be understood as definite in context.

من أجل means for the sake of, for, or in order to.

In this sentence, it introduces the purpose:

من أجل سكب الصلصة = for pouring the sauce / in order to pour the sauce

So the sentence structure is:

• يستعمل صديقي كوب القياس = My friend uses the measuring cup
• من أجل سكب الصلصة = in order to pour the sauce

Arabic often uses من أجل when it wants to express purpose clearly.

Here سكب is a maṣdar (verbal noun), meaning pouring.

After expressions like من أجل, Arabic often uses a noun-like verbal form rather than a full conjugated verb.

So:

• من أجل سكب الصلصة = for pouring the sauce
• literally: for the pouring of the sauce

A different structure is also possible, such as:

• لكي يسكب الصلصة = so that he may pour the sauce

But in your sentence, the maṣdar structure is what is being used.

الصلصة has الـ, so it means the sauce.

Without الـ, صلصة would mean a sauce / sauce in a more general sense.

In this sentence, the speaker is referring to a specific sauce, so الصلصة is definite:

• الصلصة = the sauce

Also, in سكب الصلصة, the phrase works as another iḍāfa-like structure:

• سكب = pouring
• الصلصة = the sauce

So literally: the pouring of the sauce.

If you add full case endings, the sentence would be:

يستعملُ صديقي كوبَ القياسِ من أجلِ سكبِ الصلصةِ.

Here is what each part is doing:

• يستعملُ — verb
• صديقي — subject: my friend
• كوبَ — direct object: cup
• القياسِ — genitive because it is the second part of iḍāfa
• من أجلِ — prepositional phrase
• سكبِ — genitive after من أجل
• الصلصةِ — genitive as the second term in the iḍāfa with سكب

A learner reading normal unvowelled Arabic will usually not see these endings written, but they are still part of the grammar.

Because صديقي ends with the possessive suffix ـي (my), and that often hides the normal visible case ending.

So even though صديقي is the subject and therefore nominative, you do not clearly see a final ـُ the way you would on a simple noun.

This is normal and common with nouns attached to ياء المتكلم (my).

A careful pronunciation would be roughly:

yastaʿmilu ṣadīqī kūba al-qiyāsi min ajli sakbi aṣ-ṣalṣati

A few helpful notes:

• يستعمل = yastaʿmilu
• صديقي = ṣadīqī
• كوب = kūb
• القياس = al-qiyās
• الصلصة is pronounced with assimilation of ال to ص, so it sounds like aṣ-ṣalṣa

That last point happens because ص is a sun letter.

Yes. Another very common verb is يستخدم.

So you may also hear:

يستخدم صديقي كوب القياس من أجل سكب الصلصة.

Both يستعمل and يستخدم can mean use.
Very roughly:

• يستعمل can sound a bit more formal or classical in some contexts
• يستخدم is extremely common in modern usage

Both are correct Arabic.