Can you imagine a way a group of quadruplegics ( imho a good aproximation of a stranded dolphin with a human brain - except that their skin does not dry out - ) could fell a tree with stone tools?

I don't know about quadruplegics, but I can imagine a way that a group of dolphins might be able to fell a tree using only stone tools and a bit of seaweed.

First, they would need a suitable tree. One that grows near the water (probably near a river they can swim up) where it's easy enough to get to (and other dolphins can stay in the river and splash the woodcutter to prevent his skin from drying out).

Then, they need a stone axehead. This can be made, fairly laboriously, using only stone; chipping away until is is the right shape and sharp enough.

The dolphins then elect one of their number to be the woodcutter, and use the kelp to tie the axehead to his tail, at a carefully chosen angle. (This part can be done underwater, where the dolphin(s) tying the knots can swim around at all angles to get the kelp in position; another dolphin would probably need to hold the axehead in position while this is going on).

A dolphin's tail can certainly swing back and forth (or, up and down) with some force, as this motion is used when swimming. So the woodcutter would then need to climb out of the water, turn on his side, and strike the tree repeatedly with the axehead...

If he has a better idea of what he's doing than I would, he may even be able to arrange for the tree to fall into the river, at which point transportation is comparatively easily handled.