I tend to agree with your friend's assessment of Ti and steel. I think Ti is an ideal material for smaller and lighter riders and that bigger riders may find it lacking. It's true what was said above that Ti bikes can be made stiffer using larger tubes - but not entirely true.
Most of the tubes on a bike have no practical limit to the diameter of the tube that can be used so it's easy to design and build the frame with large main tubes - but the issue boils down to chainstays. Because we need to squeeze the chainstays into a small space left between the chainrings and the tire the size of the chainstays is limited to about 7/8" in most cases. This is what is used on most steel bikes and getting anything larger in their is near impossible without some serious stiffness stealing dents to give clearance. So when you combine this with the fact that Ti is much more flexible than steel you end up with the rear end of the Ti bike being much softer than the rear end of a steel bike.
Some will counter by saying that you can use ovalized Ti chainstays to get more material in there to make it stiffer - but it has little effect. If you make the Ti stays 20 mm wide and say 30 mm tall (a big assed stay!) it will not be as laterally stiff as a round 22.2 mm steel stay. This is for two reasons. First is the obvious that Ti flexes much more than steel does. The second reason is that the oval stay is still not as large in diameter horizontally that the steel stay is. Any oval tube has the bending stiffness roughly equivalent to a round stay of the same diameter. So an oval Ti c-stay will behave in lateral flex about the same as a round Ti tube whose diameter is the same as the minor diameter of the oval. ........... I don't think I said that well. How about this? - a tall oval Ti c-stay that is 20 mm wide will flex about the same as a round Ti c-stay that is 20 mm in diameter.
So you end up with a oval Ti stay that is stiff vertically and about the same laterally as a round Ti stay. There is very little to be gained here with diameter as long as the chainrings and tire are competing for space. But when you look at a steel stay you have the same amount of room so can fit the same diameter c-stay in there and because it's steel it will be MUCH stiffer.
You will also see folks argue that if you put a big down tube in the bike that it will be stiff. It doesn't really work that way IMO. There are only two tubes that can and do transfer the power from the BB to the rear wheel and those are the chainstays. You can remove the down tube and make a bike that transfers power very well - i.e. the "Slingshot" bikes what have a cable instead of a down tube. The Slingshots had very stiff BB's because the chainstays were designed to do the job. Since the chainstays do the work of getting the power from the BB to the rear wheel they are the tubes that will make or break the power transfer of the bike. The chainstays see a combination of three loads - compression end to end, torsion, and bending. The compression loads are small and can for the most part be set aside while the other two are very dependent on diameter. And since the diameter that can be used is limited by the space to put the tube in then you really need to have a stiff material to get the job done and Ti just isn't stiff enough if you are a big guy.
This is the reason I think Ti isn't great for big guys and that other materials like carbon or steel can be better. IMO big guys would be best off getting a steel or carbon bike designed with a big guy in mind and leave Ti to smaller and lighter riders.
Thanks for reading all the above - I got wordy.
Dave