You are right to be worried and I would advise you to take this seriously, they have already crossed a line and these things can escalate fast.
I think you need to sit down with her and ask her to tell you honestly how she feels. Also tell her how much you love her but recognise she is her own person.
I would be inclined not to tell her about the tablet (yet) but simply say, calmly, that you know something is not right and that you would prefer her to be honest with you here and now.
A lot at this stage depends on her reaction. People can have crushes, we are human. If she is honest, you are in a better place and you should try to thank her for that and agree a way forward. Hopefully she will think how much you mean to her, but either way honesty is critical now. If she is not honest, I would be inclined to confront her with the evidence.
She may naively think that meeting up will be innocent, but I can assure you he won't see it that way. Deep down she probably knows that.
The worst outcome is denial from her and you being gaslighted.
Be strong, this is hard, but you still have a chance. Good luck.