Does Onesiphorus tell Wilhelm to come with them? If not, he will stay behind and continue to check out the tree, feeling it, looking for any small branches or any that have fallen from the tree.
If he finds some small branches on the tree, he will try to break one or two off...he doubts that he'll be able to, but this is definitely a bizarre tree. Ditto with any fallen branches that look like they came from the tree. If he finds any, he will try to whittle on it for a few minutes to see what happens (carefully).
He will also keep a careful eye on the fire, make sure nothing happens.
If, on the other hand, he is ordered to come to the covenant with the magus and the woodsman, he will, sharing any observations he made about the tree and asking Onesiphorus's "expert opinion" on the matter.
Onesiphorus will invite Willhelm to travel back with them. Examine any branches that can be found and look for a sappling that could be dug out and returned to the covenant. The branches and twigs that could be found will be investigated in the laboratory.
A collection of branches is found and taken back to the covenant. Saplings collected may or may not be a descendant of the tree. They'll have to be grown for a while, or a herbam specialist needs to grow them faster, to find out.
Examining the branches finds that they are all unremarkable. It is unlikely a full season of investigation in the lab will discover any unusual properties.
I'm closing this story out. If Mad Max comes back, he can reopen his investigation at at that time.
Willhelm can have 10 xp from this story. My reasoning for this is that Willhelm's been asked to provide for the needs of the man's family, while the woodcutter recovers. Hunting, survival, bargain, anything related to securing household supplies and ensuring to the care of his family will be put on Willhelm. If you come up with an ability that I haven't listed that you'd sink XP into, let me know.