Newly bought IPhone 13 pro

It should fine.

Unless your friend does not use Air Conditioning and his apartment is like a Sauna, or is storing the phone somewhere not air-conditioned like an attic or a garage, it should be fine.

In New York most if not all apartments have air conditioning. The Battery should not suffer any performance loss, or capacity loss in 2 or 3 months.

It will lose a bit of charge, and be lower than the normal 70 to 80% charge iPhones come pre-charged at, but should not affect the function of the battery at all.

It should not be a real problem to keep it stored for 2 or 3 months. Do note however, that the warranty while only paid in the U.S. will start at the time of shipment form Apple, and not when you activate it.

Beyond that, you may find that the iPhone battery is lower than normal, though 2 or 3 months should not affect it too much.

Adding to Phil0124’s remarks, we don’t know where this iPhone might be stored for all those months.

For example, if it’s stored some place with temperature in excess of 113°F (45°C), its environmental requirements have been violated and degraded performance should be expected.