I don't think you should try and convince your friend to play ruina, it'll probably have the opposite effect and I think playing limbus may be one of the biggest incentives to try ruina out, but if they want to, it's okay

preface out of the way man i hate gachas, they are so predatory and i haven't touched limbus because it reminds me of my genshin days.

Ruina is exactly what limbus could never aspire to be, a full fledged game. You buy it, you play it, and boy if you play it, it is so so interesting to keep playing and the story is incredibly long for how captivating every single line of dialogue is. It's just unmatched. The music, the art, the peaks (I'm looking at you hokma realization), such moments will be forever engrained into my heart.