Good advice, and I do believe that is the best way to resolve matters like these.
Any "political" or any "sweet talk" approach would actually prolong the matters. If they refuse to sit down and talk about it, I suggest a dojo meeting. Again, do your best not to involve your sensei.
The meeting could be just between the members, or it may be headed by the dojo-cho. Basically you need a moderator that is unbiased but at the same time the wisest of the group. The moderator should be able to see beyond what is presented.
All the members should help as witnesses and/or character witness. If you think this started to sound like a court, don't. It's an open discussion forum, where each has a chance to be heard. No judgements shall be passed. The final decision will be made only between the people that have the problem.
Use this time also to resolve any problems that might also exist in the dojo. Any problem at all. Anybody that believes that there are other problems are allowed to speak afterwards.
Do these type of forum or open discussion once in a while. This helps the cohesiveness of the members and prevents "politics".