Partner buying out friend – tenants in common or minimal share?

I’m really struggling to understand the SDLT implications for a transaction I have coming up, I hope someone here can enlighten me!

I currently own a property with a friend who I am buying out. My partner will be replacing him on my mortgage. As she owns her own property already I assume the transaction will be subject to the higher rate SDLT (although I have read a few articles that say jointly owned property that involves a transfer of one of the owners is not subject to SDLT if the property is owned equally?)

I understand that her consideration will consist of the amount we pay to my friend to buy him out plus her share of the mortgage debt. If we opt for a 50/50 joint tenants ownership then her consideration will be the amount we pay to my friend plus 50% of the mortgage debt. However, what if we opt for a tenants in common ownership with her owning a minimal share of the property – does this affect her SDLT liability?

Would really appreciate any insight/advice!

Thanks in advance

Phil