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.