Hey stop stressing, usually its the other way around!
there is such a thing as pre cum or pre ejaculation.
When I have sex with my boyfriend it is always unprotected and I find half way through his penis starts to throb. This is when usually pre ejaculation occurs when only half of the seaman is released.
The best thing that you and your friend can do is sit down and talk about all the possibilities and remember if you dont want that possibility you should take the right protection.
God luck let us know how it goes