Wall reflections will have a seriously detrimental effect on contrast and black levels.
You can show him this example:
And a detailed explanation:
As in the example above, curtains can be used to combat the unwanted reflections. Or staple black velvet to walls or frames/wood pannels.
If the room will be modified a white screen will do best.
This configuration will allow the 5050UB's to display black details without getting washed out. However, if the room will stay the way that it is (or be painted in a light grey) the 5050UB's black levels will be wasted. The Epson HC4010 is the downgraded version of the 5050UB with poorer blacks, and a 10GB HDMI chip as opposed to 18GB on the 5050 (and the HT3550).
The main difference between the HT3550 and 5050UB is the black level the Epson is capable of producing (in a properly blacked out room).
When calculating distances make sure to take into account the depth of the projector and the space behind it (for wires/power cable) .
Optimal distance to screen is subjective, but 100", even 120" from 10' with a 4K display is on the far side.
What is sometimes suggested is the projector to be used on a wall for few weeks before making a decision on the screen size.
Although both claim to be 4K, the Epson has ~4 million pixels, and the Benq ~8 million pixels.
XY screens makes a retractable ALR tensioned AT screen, but an ALR screen will very likely produce a hotspot at those throw ranges. It's also expensive.
A negative gain grey/white screen would be an option for the Epson since it has the lumens to power it, (while doable) it would be less ideal for the Benq.
Another kind of setup would be one with a UST projector if there is room to place the projector in front of the TV, where the screen would drop down. UST ALR screens exist that do not exhibit any hotspot.
This setup is significantly more expensive.
LE: The Optoma UHD51A has a more lumens and a similar throw range as the HT3550. It also has a RGBRGB color wheel just like the Benq. No iris though. The Benq probably has a better HDR tone mapping feature. The Optoma could power a negative gain screen.