If my Apple ID has a capital letter and not a small letter will that make it be in correct

The Apple ID is supposed to be exactly what the email address is which is being used to make the Apple ID. Is your friend having problems? If so, more information would help.

I know with my own, the prefix part (the part in front of the @) acts case independent as sometimes spell checkers automatically capitalize the first letter - but it won’t matter for an AppleID login. The suffix part (the domain after the @) is always not case sensitive in actual emails as well as any login I’ve ever seen that uses an email address as login ID as Apple does.

SMTP email servers are supposed to treat the prefix part as case sensitive, although that is not actually always the way they work and often emails will send and receive just fine even with mis-case letters in the prefix.