Urgent legal advice please!!

[Gettingbadagain]

DHs business partner has bowed out of the project halfway through after months and months of DH working two jobs (day job and working late into the night on side project). BP has decided to use the idea but go down a different path. Idea was initially BPs but DH has put in so much work and at least 70% of the ideas are his. Can DH continue on the path he’s on to finish the project or does he risk being sued but (ex)BP even though he’s chosen to walk away? There were no contracts/legal write ups etc however they both agreed they had an equal stake.

Thank you so much.

[Gettingbadagain]

I know we need to see a lawyer but I think we’re pretty clueless as to where to start. DH is devastated as this was also a very good friend of his.