No that is not speculation at all, I have taken a degree in Law and done a brief bit of research. A company has what is called assets which are things it owns such as pens, desks, computers, cars, machinery etc.. basically anything at all. If and most likely these baliffs have given him a notice to say that they intend to receive payment on behalf of the creditors by sezing all company assetts. They will not have to take the goods then and there, but they have to make you aware of this (most likely the form your mate has signed)

Goods, which can be seized
The bailiff can only seize goods which belong to the debtor. However, the bailiff can seize goods, which are jointly owned even if the other joint owner is not the debtor. As this is recovery truck is obviously an assett of the business and baring in mins some legally binding forms have been signed this may amount to a charge of theft should he sell it to you.

Walking Possession Agreement
This is an agreement signed by the debtor which allows the goods to remain with the debtor as long as they keep to a payment arrangement. However if the debtor defaults on a payment arrangement the bailiff can return, force entry, and seize the goods. It may have been this that was signed and I believe this to be the most likely outcome.

As b19 said the law does not work on assumptions, it works on facts. The facts here would be, your friend has sold a recovery truck which the bailiffs had recorded on most likely the walking possession agreement. This agreement states that the goods are now the bailiffs property, even though they are still being used by your friend.

I believe what you think you will get off on is the fact that you think you can prove that this receipt he gave you was before the seizing of goods, however the receipt should have been produced at the time or made them aware of it.