If fortune turns its back on a people, their enemy will suffice. --- إذا أدبر الدهر عن قوم كفى عدوهم

 That is, if he helps them, he will be able to deal with their enemy.

AI said  

There are several English proverbs that can be used to express this meaning, the closest being:

When fortune turns against you, even your friends disappear.

Or, a more similar proverb:

Misfortunes never come alone.

And for the meaning of the Arabic proverb—that if time abandons a person, they become too weak for their enemy to exert any effort—there is also:

A falling tree needs no push.

 

أي، إذا ساعدهم كفاهم أمر عدوهم.