I have no problem tailoring a cake to suit their needs as long as I can safely guarantee it, but because I make cakes containing nuts, etc. on a regular basis I cannot 100% guarantee their exclusion. Their may still be traces of the products though not actually used in the making of the cake.
If the customer is happy with this, then there is no issue (minor allergies). Should they require a cake that 100% guarantee's no nuts, etc. completely due to a very serious allergy I will not be able to do this and I will have to recommend they go somewhere specialised. I only mentioned the allergies in reference to someones comments on this thread.
I personally have no problems tailoring a cake for allergies - I have dairy allergies myself and also have a sister who is a severe Coeliac so I am very familiar with how difficult it is to find products that 100% guarantee the exclusion of certain products.
Business is business and I am not one to refuse it, I need all I can get.