Different Bookies have different ways of doing things. Most Inshop bets placed are not flagged unless the stake is £50 + (Some calculate the exposure vs' the probability based on the odds of each event as would be in an accumulator).
If he had 10 5/1 bets in the accumulater and I believe the bookies would calculate the probability of that event at being about 0.000002% which is minute and thus they probably would not take much notice of it. (If they were all even money, that would be about 0.1%)
This snow bet was a Singles only bet and so an accumulator was void. I do believe all bookies have T&C which which allows them to correct a bet, after it is placed if it is incorrect.
They could have refunded his £5 stake or as I believe they did do, they paid out on his first selection.
The best value bet I see out is on boyle sports online betting. They offer odds of 3/1 on Ricki Lambert to score and Southampton to win, its not a scorecast. Its not offered every week, but it is most weeks, worth checking out because they have been giving close to 2/1 for Tevez to score and City to win the last few weeks. That'd be a nice £20 double each week!!