I think she would say that she always knew there's something going on beetwen them :D I guess, she would admit the fact that Cuddy is competent for having a relationship with him. She may be also wondering why House tried so hard to get Cuddy, and why did he let her go easily in season 2. And i'm pretty sure that she would be amused that House is off of Vicodin and is able to feel again. She would wish good luck to Cuddy and conselhos her never ever make decisions about House's leg ;)
I'm sure she will be happy for him, although I think it would depend on how happy she is with Mark. BUT it is going to STING !!! She was going to leave Mark and go back to House, remember he is "THE ONE." And she admitted to not being over him. She may wonder why he felt he could not make her happy, and why he now thinks he can be happy with Cuddy !!