Learn English – Idiom for “don’t put me with somebody together in the same sentence”

2 sentences that comes to my mind are:

Physician, heal yourself! 1

and

The shoemaker's children go barefoot 2

Both are referring to people who are able to help others, but have problems help themselves. Although the situation is a bit other, because in your case the person that have helped you got the same problem not in the same time, but afterwards.

Disclaimer while in first case I'm sure it has the same meaning as in my languange, in second case in Polish the saying is referring to the shoemaker going barefoot himself.

Consider the phrase rush to judgment as the action of one who concludes too quickly on the basis of insufficient or misconstrued evidence.

In an article published at stubbornthings.org Jerry Richardson contrasts a rush to judgment with the concept of the Rule of Law expounded by Madison in Federalist No. 51:

Vigilantes often rush to judgment, and get it wrong. Result? Innocent people get killed or otherwise unjustly punished.

I prefer this expression as generally accessible to less cliched than "judging a book by its cover."