I don't advise getting the other male neutered, for a couple main reasons. The first being not all chins like all other chins, so you could put the male through that whole ordeal for nothing. It's not advisable to allow the male and female to be together even just to see if they get along, mating takes seconds, faster then you can stop them. Also even previously bonded pairs can have the bond broken in the time it takes the male to heal from neutering.
Second I don't know if you had the previous male neutered or he came to you neutered but neutering is not that safe for chins. They can die being prepped for surgery, during surgery, or have post surgery complications and die. You will have to keep him separate from the female for 12 weeks to make sure he is fully healed and all the sperm is out of his system. That is assuming nothing goes wrong, it's not uncommon for you to need to hand fed him do to pain, as well as watch him pretty much 24/7 for the first couple weeks at least to make sure he leaves the area alone. It's not a simple surgery like it is for cats and dogs, the testicles are internal so it's more like a spay then a neutering.
A much better idea is to get the female a cuddle buddy, which is just a chinchilla shaped fleece toy.
https://www.etsy.com/listing/79459557/chinnie-buddie-for-chinchillas-pocket Or if you want another chin, get another female to try to bond with Plum instead, then you don't have to worry about risking your other chin's life neutering him.
Chins do grieve though, and it can kill them (they stop eating and drinking, don't move around enough, etc) so make sure you give her lots of extra attention and make sure she is eating and drinking enough. If she is too depressed to eat (keep an eye on her weight and watch for loss) you may have to encourage her to eat or hand feed her until she is feeling better again. You can also try putting some new toys in the cage, or at least rearrange the cage so it's new looking to get her mind focused on other things.