I am a little late to this, but have experienced a similar experience when my trans-son came out and his new friend (also a trans-male) was his new boyfriend. Similarly, his parents did not know he was trans, however, they spent most their time at my house. I found the best thing we can do as parents is talk it out. Talk to them about your concerns, ask them questions, and try to be open-minded. Also, it is not our place to out anyone, but perhaps refrain from sleepovers without your supervision? I’ve found that allowing open communication, without judgement, will allow your child to trust you with bigger things later. Hope this helps!