Most people don't realize that its easy to arrest someone, even on BS charges. The real issue is whether the Commonwealth can support their case. The mere fact that someone dropped someone off in a certain area and later asserted their constitutional privileged would not in my view be probable cause to arrest. That doesn't mean the police won't arrest.
This information does not create an attorney /client relationship and should not be use or relied upon to make any decision in your case. Only consultation with your own attorney can provide you with the advice you need for your case.
As noted already the more important question is whether the Commonwealth can substantiate the charges. An illegal arrest only has practical significance is evidence in some form is derived from the illegal arrest, such as an inculpatory statement or recovery of physical evidence from the accused at the time of the arrest. Qualified counsel should be retained at the earliest possible time to maximize the benefit of having a good attorney.
An attorney would have to review all the facts before rendering an opinion. You may not have all the information that was presented to the police at the time of the arrest. The person taken into custody needs an experienced criminal defense attorney to determine whether the police acted improperly.
What difference does it make if it is an illegal arrest? The defendant gave no statement and there is, as far as you have told us, no evidence that was seized pursuant to the arrest. So there is nothing to suppress. The illegality of an arrest does not preclude charge, prosecution and conviction. So where are you going with this?
I agree with my colleagues. What matters is what evidence the prosecution is ultimately able to assemble. This may be a difficult case for the state to prove, but you can assume that the police are still looking for more evidence.
The defendant should continue to keep his mouth shut. Friends and rappies are more dangerous than detectives.
If you are saying the only evidence against him is 2 people who saw him drop the robber off, i'd say, perhaps there would be enough for an arrest, maybe. However, the case shouldnt get past the preliminary hearing stage and definately should be a defense verdict at trial. However, my opinion is based on only what you are saying, and i don't know all the facts.