Yes, it's hard to convince common ppl to use even Protonmail or Signal, and PGP is out of the question. I bet your friend won't be happy to even that password protected mail, so eventually you'll use Proton w/out E2E encryption w/ them, but incoming messages will be encrypted before it is stored in Proton's server so there's still a little benefit. Even if you could persuade them to use Protonmail, it's likely they use poor password so all your communication can be compromised even by script kiddy. So I also use Proton like you mentioned. Make sure she and you use strong password, ideally 110+ bit of entropy (around 20 char including upper + lowercase, numbers and sp char).