Peer-review: Reference to unpublished work Paper I am reviewing

I am reviewing a paper from authors Xs. In their paper, they assume that a function f was equal to g. In my opinion (see below paragraph), the particular choice of the function f can have important impact on the end results. I would note that most people in the field assume that f is equal to g (without good justification for this assumption).

Unpublished work from a colleague

A colleague of mine (let's call him Y) had the idea that depending on the field of application the choice of f might be important. He worked on it and we recently talked about his first results which suggest that he was right; if f equals to h for example, the results of interest might differ. He has not published his work yet.

Issue

I would like to suggest to authors Xs to comment on the potential importance of the f they chose. However, by doing so I would give author Xs the credit of being the firsts to recognize the importance of the choice of f while it is Y who first came up with this idea. I obviously want to avoid that. If the work of Y were published, I would just suggest Xs to cite this work but it is not published.

Should I just remain silent?
It seems highly unethical to disclose the work of Y. If no previous literature questions f=g, I don't see your role as the reviewer to question this. Hold your horses, review this in view of past literature, and wait until the work of Y is public.