Sorry man, my post was just a light jab. Also first, as JFG said diet is most important. Especially if you are 15-20% body fat and have a lot of fat to lose.
What I was trying to get at is "which" 5x5 program, how heavy or what percent body fat is the person and what is their goal, and how athletic/proficient are they in the movements?
Look at the programs conceptually: 5x5 tend to accumulate a lot of fatigue and volume in high intensity ranges once you have run them for awhile. My opinion, but this won't work really well for fat loss unless the lifter has a lot of room to grow in the movements. The more proficient you are the more taxing it will be.
You're not pushing the intensities as much on the 10x3 so it will help keep you fresh from workout to workout. If you want to try to push your strength gains you could ramp the last few sets too (W1 last 3x3 @ 80, W2 last 3x3 @ 85, W3 1-3 sets x 3 @90)
If you still have a lot of fat to lose and your lifting numbers aren't that high yet just focus on the diet and keeping a balanced program that doesn't leave you feeling like death after and between each workout : )
(Oh and HIIT I would personally do on off days and follow it up by a brisk 20 min walk or something. If your workout was taxing and you try to do HIIT after, you likely won't be able to truly push to max intensity during the HIIT)