Twitter is really sensitive with login Locations. I had that same problem with clients of UK (My IP is from Spain)
You and your client must use the same proxy(private unique proxy, not shared)
If you do not want to have problems with bans the best way is avoid logins from outside. So you will do all the stuff.
Remember: MP can also post a list of twitter post with images so you only have to add on a Campaign. Hope that helps