you cant take responsibility for any job involving gas, the name on the paperwork is the other guys and it is his bum on the line for any issues, no matter what you believe. As far as nicking your customers goes, you lost this one and he gained one, part of business,but ultimately he won't get any more from you. if you approach the customer and point out it is your job and you are both doing the work, do you believe the customer will see your side at having to pay for 2 when they have one person happy to do the work. As far as your pricing goes, far to low imho, if you are hacking out floors and removing units it takes time, your idea of 22mm gas runs may well be totally out and need 28mm, so you need to price up to ensure you can afford to do ther job in the first place