The cost of an SLS launch is significantly more expensive then a Falcon Heavy. Could an Orion spacecraft be mated to the Falcon Heavy to save launch costs?
Based on weight to LEO, the falcon heavy could lift the Orion.
The current fairing OD is 5.2 meters and the Orion capsule is 5 meters wide. Aerodynamically it'd be pretty similar to the standard fairing.
Another issue would be the launch abort system. NASA typically wants these so SpaceX would need to include it and make an adapter to step up the diameter of the falcon 9 core.
One of the challenges Boeing has faced with CST-100 was the aerodynamics of a larger diameter capsule on a skinnier stack. This would be similar to the problems encountered placing Orion on Falcon Heavy, and would require aerodynamic studies to determine if it is stable, and what modifications might be required.
Besides the windtunnel and simulation tests, the government may require a test of the inflight abort system since A) the current system (tested standalone) was designed for SLS and B) They are aware SpaceX could realistically perform this test much cheaper than other providers, due to cheaper launch costs and possibilities of reuse (a cheaper used booster, and maybe saving the booster).