Critiquing or commenting on someone's talk
X, being an expert in the field, can presumably find ways to communicate their opinions to the speaker, publicly or privately, if they wish to do so. 

Often, opinions communicated between experts will be much more complicated and mixed than a simple like/dislike. "I agree with you on points A, B, and C, but I think point D is contradicted by Y's research. I'll send you a reference." is much more likely than either extreme.

If you just heard X talking about point D and Y's research, you could get the impression X disliked the talk. If you just heard X talking up points A, B, and C you could get the impression X liked the talk. Neither impression would be fully accurate.

Unless X has positively authorized you to speak on their behalf, you should not do so. If asked directly, indicate that you are unsure and, as suggested in a comment, encourage the speaker to talk to X directly.