What should I do, as a reviewer, if I see another reviewer suggesting the authors to cite papers not relevant to the manuscript?

I would recommend raising your concern to the editor. If the editor is honest, they will not ask the authors to follow the citations en masse.

Given that the citations are all in the same journal, however, it is possible that the editor is part of the same citation cartel and may blow off your concern. In this case, it may be worth raising the concern to the publisher and/or one of the major indices. Citation cartels have been gaining attention in recent years, and if this is part of a systematic pattern of coercive citation, then those larger organizations may be motivated to investigate and sanction.

My answer depends on where in the review process the manuscript is.

If you believe the manuscript will come back to you for a second round of reviews, I would wait until then. At that point, see how the authors handled the issue. If they included irrelevant citations, tell them in your review. Something along the lines of:

While reviewer 2 suggested you cite everything ever written by Smith, they do not seem to be appropriate in the cited context.

I would then add a confidential note to the editor saying that they may want to comment on if the cited work is needed.

If you believe the manuscript will not come back to you (either because the authors will give up or it will be accepted), then you may want to tell the editor directly. I tend to defer to the editor on these issues, so would not say anything.