There's a difference between "explain" and "excuse".
I can understand why people feel angry, that they're not being listened to, etc., and the reasons for that are many fold and go back 2-3 decades or more. Whether they're picking the right target/reason for their anger is a different matter.
But despite understanding "why", i.e. the explanation as to why people feel disenfranchised and angry, it doesn't excuse what rioters are doing. Rioting is never an acceptable solution, however aggrieved someone feels, however angry they are, there's never an excuse for rioting.
Re the OP, I don't see there's anything wrong with their partner "understanding" why people feel as they do, from her comments, he's not condoning them rioting nor feels inclined to riot himself.
It's important that we listen to and understand everyone's point of view, even if we don't agree with them (or in the case of riots, think the rioters are low life scum as I do!). Even though I think of rioters in that way, I can still, kind of, understand where some of them are coming from when you see the state of the country, lack of amenities, crap roads, poor public transport, lack of jobs, etc etc. But rioting is not the answer!