Your girlfriend sounds awesome! If she's going for a sludge metal sound, I'd assume that 5th string is a low B?
My main bass is a Rickenbacker 4003 https://flic.kr/p/bE7D5v that was 13 years in the making. It was the very first bass I ever played, and I lusted over it until I finally acquired it after getting promoted at my old job.
I also play a 6-string Spector bass that's a ton of fun to play. I used to play in a three-piece band, so I had to cover a lot of ground. The extra strings allowed me to play chords like a rhythm guitar and have a lot of fun doing a bunch of huge runs over a bunch of octaves.