It is free, and yes the bus stop is on the corner by the Ariel. The only thing to watch is getting to the correct bus stop, when departing the hotel, it's at the diagonal opposite to the hotel and may take a few minutes to reach do due to the phasing of the traffic lights. The bus stop has a dot matrix display to indicate the bus is going to Heathrow. There are also usually a lot of people at that bus stop - working at the airport - since it's an interchange bus stop from other routes.

From Heathrow it's more obvious and there usually an announcement made that the free bus zone is about to end at Harlington Corner. Some buses turn the corner at the hotel, some go on down Bath Road, but the stop and hotel are easy to work out, just tell him to look leftwards for the big green H. At Heathrow you want the second last bus stop in the series of bus stop at Heathrow Central, again usually a lot of airport workers at the stop concenred.

There are a lot more bus services than the two you mentioned.