“When is it permissible to discuss others’ problem (if ever)?”
If you truly are going to help the person with what they need. You Will look for a shidduch for them. You Will go out and collect $ for them. So and so on.
Yes, unfortunately many people enjoy speaking about others, which is pure LH.
Ex. -Even though it’s innocent -how many people when asked what is with so and so, will answer -Oh, he/she is divorced? Who gave them the right to spread this around, unless they are really trying to help the person. If the person asking is just curious and isn’t really going to make any effort to help, then the correct response is – “I don’t really know what is going on with so and so”!