Absolutely!!
From what you wrote OP, it sounds like you have seen this new nanny w/your friend's children when your friend hasn't been around. If there are some traits that she has that are not too enduring at this stage, trust me....It
will not improve over time.
Sounds like your friend needs to re-consider her decision.
While it would be ideal for her to re-hire the former nanny, if I were the former nanny, I would say "Screw you!!" to your friend after being fired for my weight.
That's just me though.
But to answer the question...Go right ahead...You need to let your friend know this new nanny is a doozy.