He could also cut some plywood pieces to fit the tube very snugly and fiberglass or epoxy them in place or something similar to stiffen it up without welding. Obviously, I think welding is the best choice but I disagree on the word "MUST" because there are a few other things that could be done there and still leave room to bolt them down. The welding would only take a few minutes, so in my opinion, it is easier to find someone who has a welder to help out, so I think we agree on that.
I assume you are talking about the gantry when you say Y axis. Because the span is small, ribs are not necessary. However, if he wanted to add ribs, they could be added just past where the bolts go. I welded ribs into my build using a stick welder duct taped to a broom handle. Also the gantry on my build has bolts at the ends, in the tube, and I put the two end ribs past where it bolts on. My build is a moving gantry. It worked out fairly well believe it or not. 1Jumper10 used a different technique to weld ribs on his build. Even snug fit plywood that is glued in place can be used. I looked at a large commercial router in a different thread, 9mm wall, very large span, no ribs. There are a few options available if he wants to add ribs. It won't make any difference worth mentioning if the ribs are just past where the bolts go as opposed to end caps if he decides to do it that way.
Oh thanks. . I hope it helps out the OP with his progress. Perhaps he will use some ideas but modify it to be better.