Find out if they use a carbon pre- or post-filter with their reverse osmosis system, and if their colonic equipment uses a UV filter. If they answer 'yes' to both then there is no problem. If they do not then there MAY be a problem regarding organic components such as bacteria, viruses, parasites.
Frankly, I don't know much about filtration systems for well water... it's not an issue in Chicago where the chlorine levels are through the roof! But I did visit a colleague at her colonic center who had well water, and she had multiple filtrations systems on her equipment. She had to replace filters frequently...so maintenance is also an issue. She has had no reported problems with this system.
Lastly...and most importantly...trust your intuition! If it doesn't feel right to you then don't go there. Especially since this is your first experience with colonics you should feel as comfortable and relaxed as possible.