Building to Flip - Contractor works for free

Heres the situation.

A partner of mine has been looking to build to flip a luxury home in Florida. While interviewing contractors to build to shell on a recently acquired lot of land (we have our own subs for interior build), he got a proposal to partner with one of the Contractors.

 The deal is invoice material payment only and splitting the net profit 60/40.

He wants to bring me onto his current job sites and will provide references.

Neither one of us had considered this an option before. Is this common? Are there particular risks associated with this method I should be aware of?

Right now it seems like lower up-front costs and motivation for the builder/contractor to perform quickly and well enough to maximize ARV.

Any thoughts would be appreciated!