Partner has shared mortgage with ex wife - but we want a joint bank account - will this cause issues?

My partner has been divorced for sometime now and despite having a shared mortgage with his ex wife, he is sole payer of this and comes out of his bank account.
She has made clear she does not want responsibility for this and has distanced herself from the house for the past few years. He is happy with this and does not want any money off her as she doesn't live here.
Only reason she remains on the mortgage is it is in negative equity and will need to keep paying towards it until a time we can have her name removed and transferred over to us.

We have been together for the past few years and now talking about future etc and joint bank account has came up.
I (to the best of my knowledge) have perfect credit score, I don't believe in credit and any loans I took out to improve my score are paid off in under 6 month.
His rating is ok, because of situations with his wife and he cosigned a couple of things for her which she couldn't handle, his is slowly building up over the past few years so I assume he getting better.

We want to have a joint bank account do its easier to handle our wages but someone said shared mortgages can cause issues financially.
I am wanting to know if because she's linked to the mortgage, can anything she gets into trouble financially cause issues for my partner despite being divorced and do not have any other debt together besides the mortgage and can this cause issues for us getting a joint bank account.

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