Besides you can pay only using paypal checkout without linking your paypal to fiverr.
Yes, you can just use your account instead of him on buying stuff there. But i see the only reason why op is doing this is because it will give his fiverr account a review from the seller (well, optional) and make his account more mature with at least one activity.
Plus new account (if ever his account is new) have some discount on its first purchase.
Well, if ever you have linked paypal for easy buy, you can report it immediately on paypal for dispute and remove the access manually on its settings.
You cannot ruin someones fiverr too unless you (the bad actor) made an order using your money (if there's no balance obviously) giving 1 star review to seller then seller gives you 1 star review too. I see no other things to ruin soneones fiverr account aside a seller gives a 1 star review though.