Unsure if this is allowed but don't know what other subreddit to go to

Electrical solder is way too soft, and the result will be even weaker than superglue. Depending on what it's made from a zinc or white metal solder, or 'silver solder', might work, but that requires a different technique (i.e. not a soldering iron) & it'll be a bit tricky to do without putting too much heat into it & either melting the charm or damaging the opal.

edit: Easiest/strongest solution without using jeweller's techniques and skills is probably to cut a backing piece out of suitable metal, and glue both pieces together at the joint and to the backing piece with a thin, filled (e.g. silver or aluminium-loaded) epoxy.

They didn't buy it because it's worth money, mate. It's a souvenir. Some people actually find value in little tokens of places they've been

The opal isnt worth either , Its very thin opal slices.