What is your concern about forward bends for a larger-bodied person?
Forward bends are not contraindicated for a larger body any more than they would be for a more slender body.
If your client has lower back concerns, such as bulging or herniated discs, that is of course a different story.
If your concern is your client meeting flesh on flesh compression (belly to thighs, thigh to thigh etc), then there are several things to consider when guiding Yin poses and offering modifications/alternatives.
Opening the legs makes space for belly, so in caterpillar (for example), allow your client to have her legs more parted.
Crossing or intertwining body parts is a challenge with larger thighs/calves so shoelace is extremely difficult to do - offer Square/1/2 Square as alternatives because the open space is way more attainable (and also allows for belly space. Gentler versions of 1/2 square is right ankle on left shin/top of ankle instead of left thigh OR even bringing the foot inside the leg (as in Janu Sirsasana). Tangled Roots reclining twist is problematic for the same reason, but you can take a supine swan type pose (lying on back, ankle over knee) and have them scoot their hips to the opposite side and then take that pose into a twist.
Supine Swan makes a great alternative to traditional Swan IF your client has trouble with traditional swan (some of my clients love the traditional version).
In poses where a client is lying on their back, often with legs extended they feel too deep of a lower back arch - this is often the case if one has a rounder/fuller bum which lifts them higher off the floor. Options are to put a bolster behind the knees, inviting them to lift hips and gently draw tailbone down OR offer that they can do that supine pose lying on the back with bent knees, feet flat.
I encourage my clients to lift the belly when folding forward to allow for a deeper bend.
Sometimes it's helpful in forward bends to take a bolster to top of thighs and drape arms over it and then fold forward if the client wants to be up higher but still supported in a fwd bend, or turn the bolster lengthwise, brace one end into mat or legs/feet and rest forehead on the other end.
In general, there is no reason a person's larger body should in any way prevent them from practicing Yin yoga unless they have an actual condition that is contraindicated (which is the same criteria we'd use for smaller-bodied students).
For what it's worth, Yin is very popular among my larger bodied students. The trick is to look for ways to make the poses/shapes work for each student's individual body.
I do offer workshops and trainings for teachers on modifying hatha, yin, and restorative yoga practices to larger bodied students or those with limited mobility or flexibility.
"Beauty comes in all sizes. So does confidence. Love your body." ~From The Body Positivity Yoga Manifesto