They can be mixed, they shouldn't cause any physical negative side effects unless you have heart disease or other cardiovascular problems. But if you'll have a good experience, well thats another story..
Just use your head for a second here... when you are under the influence of hallucinogens, let alone a powerful one like LSD, your mood is already altered and unstable. Meaning, you can fluctuate between extreme feelings--anxiety and calmness, euphoria and dysphoria, awe-inspiring and being frightened. Especially in inexperienced users.
So adding cocaine to the mix can worsen this cycle--it can mess up the usual course of peaking into a unifying, euphoric experience... and tapering off while you try to piece back reality and put the trip into perspective.
Plus, crashing on cocaine and being all amped out wont put you into the best of situation if you happen to be having a rough trip. Cocaine produces anxiety, especially in high doses; which is something you definetly don't need on psychedelics.
So bottom line i suggest against it. Unless the person using is extremely experienced with both substances; and doesn't mind adding a bit more paranoia and edgy-ness to the trip.