Both BA and SQ use the once cutting edge but now frankly decrepit Aérogare 1, on different Satellites (the central terminal is a doughnut - you go on travelators underground and pop up at one of 6 satellites around the doughnut.

If you have checked luggage, I wouldn't recommend waiting airside - there's been a number of reports of theft from the CDG1 belts. If not, you could go to the SQ satellite (I think *A use Sat 3, but I could be wrong - BA use Sat 5), but they've changed the security arrangements (rather hastily, so there's chipboard and glass everywhere), so there's not much to occupy yourself with when you're waiting.

Otherwise, you could arrange to meet at the bar among the duty free shops (in the doughnut, but airside). Failing that, if you do meet landside, be careful - there are multiple exits from customs!