Not really, you aren’t responsible for what other people do without your knowledge and you’d have solid evidence that someone else was able to use your network.
Sure, if your friend was committing some horrible crimes it might end up being slightly inconvenient for you.
If your "friend" uses it to download CSAM, you're in for a world of trouble. I don't know that none of my friends aren't into that stuff, but sincerely believe the friends I share a tailnet or two with aren't doing that with my connection. Tailscale has Mullvad VPN exit node integration for sketchy shit, this is more about getting access to eg Swedish television for a friend's girlfriend who gets homesick every once in a while.
Not really, you aren’t responsible for what other people do without your knowledge and you’d have solid evidence that someone else was able to use your network.
Sure, if your friend was committing some horrible crimes it might end up being slightly inconvenient for you.