Great. I've cooked a few tri tips on weber gassers as friends and family have them. This is a little long so here is the summary:
1. find the cool indirect spot
2. smoke tube burning over the one lit burner
3. cook indirect until 120F internal, Pull and wrap with foil while heating for sear
4. cut tri tip along the seam
5. Sear , rest, carve across the grain enjoy.
The key is finding the cool spot of the grill when setup for indirect heat. It is different on different grills and depending on the condition of the burners. Most weber gassers are 3 burner grills
Some weber gas grills have controls on the side and the burners go left to right, and the coolest indirect location is to use only the front burner, and cook on the right side rear. The left side has a tube called a crossover tube that carries the flame to light the middle and rear burners, and this makes the left side a little hotter.
Some weber grills have controls on the front and the burners go front to back. The coolest indirect location is to use only one outer burner and put the meat on the other side leaving the center burner off.
< if your parent's keep up on grill maintenance, this section can be skipped, but, ... >
What I would do for a grill that is not mine is give it a good cleaning. remove the grates, remove the flavorizer bar, scrape down any grease into the bottom pan, then remove the bottom pan and scrape it all into a trash can. Re install the bottom pan, and light the grill with the flav bars and grates not yet put back in. This will let you inspect the burners are all even. If so you are set. If not there could be two things to look for, a clogged burner or a cracked burner. a clogged burner might just need a good cleaning with a stainless steel brush. A cracked burner should be replaced but if you don't have time you can avoid it for the indirect cooking and use the hot spot for searing.
After you've inspected and cleaned the burners, shut it off, replace the flav bars and the grates, fire it up and get it hot for a clean off. You'll also see how quick it gets up to temp when it gets to searing time.
then I'd shut it off, let it cool down and get ready for the indirect cook. ( note you can skip all this if you parents are on top of their grill cleaning and maint) I recently cleaned my Mom's for her and it was needed.
< end of cleaning section >
When it gets to cooking time, I think you'll find one burner is more than enough for low temp indirect cooking and I'd use a smoke tube. Put the burning part of the smoke tube over the lit burner, install an indirect meat probe and smoke/cook on the cool spot until internal temp is 120F to 125F. I then pull the meat, wrap it in foil while heating the grill on high. I'd try to get the grill to at least 450F for searing. Some will easily go to 650F. If the grill has a "searing station" use that burner too.
When I reverse sear tri tip, I cut it in half along the seam. This leaves me with a larger part, and a thinner part. The larger part often needs a bit more of a sear to get it to serving temp, and having the pieces separate lets me control the done-ness separately. It also lets' me sear the cut section first.
Also, some like the tri tip done more rare or med rare and with two pieces I can sear half longer and harder and get one to Medium while leaving the other half rare.
Good luck !
I'll try to post a pic or two of where I cut the tri tip for the seam.
