I think YABU in this case, you would be excluding one person based on their dietary preferences.
It isnt too hard to cater for vegans.
Roast potatoes (without animal fat) roast veg (without butter) and the just add a vegan meat substitute.
I'd recommend the meat free vegan sausages just released by Richmonds. Taste just like the usual sausages (unexpectedly!). Inexpensive and just 8 in a pack so not really much going to waste there.
(Ingredients
Water, Rehydrated Textured Soya Protein Concentrate (16%) (Water, Soya Protein, Wheat Protein, Salt, Soya Bean Oil, Natural Flavouring), Rusk (Wheat), Coconut Oil, Soya Protein Concentrate (4%), Chicory Root Fibre, Less than 2%: Stabiliser: Methylcellulose, Natural Flavourings, Yeast Extracts, Herbs, Spices, Spice Extracts, Flavourings, Colours: Beetroot, Safflower, Salt)
For gravy beef flavoured bisto is vegan safe too.
Also plenty of pud options in the free from isle that you can buy for one person.