Well... yes. It is hard to climb once you get behind. But it's also hard to get significantly behind.
Consider: Your friend is placed in d5, but his MMR isn't at a d5 level. With a differential that high (win 5, lose 30), it's pretty significantly lower. With an MMR significantly below is ranking, it really shouldn't be easy for him to climb. After all, the goal is to keep the rank in tune with the hidden MMR.
If the system were more precise, your friend wouldn't be d5 at all - as it stands, it's simply trying to correct for a perceived mis-placement. Once it's corrected (either his rank drops to roughly where his MMR is, or his MMR raises to roughly where he's ranked), that LP gain/loss will stabilize.
It's not a perfect system, by any means, but it is effective.