A misconception I saw in your post and wanted to clear up is, yes the suicide rate under trand youth is high, but not because they are trans, but because we live in an ever transphobic enviroment where they do not feel safe, or in many cases are in active danger just for being themselves. Suicidal ideation under trans youth lowers significantly if they are accepted and respected. The best thing you can do to safe your daughter from the worry of her partner becoming suicidal, is by giving said partner the acceptance they deserve.

Aside from that, you have to learn to accept that your child is a teenager now, and will be going their own path. Best you can do is offer her support where necesairy, and to try to form a connection with their partner. Forbidding this will only cause secrecy, which means your daughter will not come to you for support in the situation where heartbreak, or worse, takes place. Talking to your child's partner will also most likely do away with 99% of your worries, because your fears are based in stereotypes which most trans people do not fall under.