damaged passionfruit stem

Manfred says...
If it isn't damaged too much (and that could mean less than about 90% of the stem cut through to the wood) then it should be OK if you just stop abusing it and look after it for a while. Water and mulch, but not against the stem. They're strong- a bit of ongoing abuse will be tolerable too.

Imagine to yourself how little cambium contact there can be on a new graft. Even after a severe run-in with a whipper-snipper yours is unlikely to be worse-off than that or you wouldn't even be asking.

As you watch it recover remember, plants don't heal, they overgrow damage, so give it time for the growth process to cover the wound and don't despair if the damaged bit keeps on looking the same, as it gets overgrown.

If it is just about cut through go out and get a new one. You can look up bridge-grafting purely out of interest but it simply isn't worthwhile except for very valuable plants. Very hard to do on such a small plant, and probably too late if it needed it. (I've never succeeded in bridge-grafting after kangaroo attack, even on quite established plants.)