You shouldn’t fire your family friends husband.

That’s not going to fall down,it’s not ideal that the wood split, but it’s not anything that can’t be repaired.

You should think about painting or staining the stringers at some point, as pressure treated wood will last much longer after sealing it. I wouldn’t go with the linseed oil others suggested. This is outdoors, and you aren’t going to want to repaint these at all, but paint or stain will probably last 10 years. Everyone wants less maintenance.

The easiest solution is probably just to add additional screws through the top, and to pre-drill them. Glueing the cracks,and adding butyl deck tape wouldn’t be bad either.

If someone steps on that, they are probably going to go through. The membrane looks solid, I don’t know if with heat or age if a hole could form. When we reroofed my garage, we didn’t leave any holes like that. Any soft spots got resheeted in plywood. Small gaps are probably okay, but that looks like 1 foot+