Could this potentially mean he's kissing someone else's girlfriend? Shouldn't it be "Li kaj sia koramikino kisas en la arbaro" if we want to say he and his own girlfriend kiss in the forest?
Yes, its ambigous. You're very close. To make this clear I think you should say "Li kun sia koramikino". (I cannot clarify, but "Li kaj sia koramikino" for me sounds just strange.)
Hm, after some thought I wouldn't say "Li kaj sia" is wrong, hm, hm, still not sure anyway. Sorry! :-P
He wouldn't be the first man to ever do that...