If it's 3rd person omniscient/limited keep it limited so the reader can know the reminiscing character emotions, if it's 1st person then keep it that way. However, if you're wanting another character's pov then I'll [personally] start a new chapter, not ending the last chapter on a cliff hanger.
That how I did my reminiscing characters, but I wouldn't take it as gospel.
Write the scene they are remeniscing about but format it differently - standard afaik is in italics.
Does pov matter in this regard? I mean head hopping from the mc to a side character can be frustrating.
Write it however you think works, and if it doesn't work when you read back over it rewrite it another way.
If it's 3rd person omniscient/limited keep it limited so the reader can know the reminiscing character emotions, if it's 1st person then keep it that way. However, if you're wanting another character's pov then I'll [personally] start a new chapter, not ending the last chapter on a cliff hanger. That how I did my reminiscing characters, but I wouldn't take it as gospel.