sorry basroil but that is not entirely correct. discounting wireless, there are three general types out on the market.
there are add-on microphones which connect up or clip on which have their own cable. these would be modmic, moovmic, zalman clip on and similar style units. this will have your largest selection of items but unless you modded them will come with a long cable so are not suitable to directly hook up to the jack provided on the headsets with a removable mic. this could work for his needs but it would involve shortening the cable or just running a second mic line down and sleeving it. given that this is the vast majority of the market and most other products are not meant to be universal or other than replacement parts, i can see why it was suggested that this is the only type of mic available as it is true from the standpoint of being standalone products.
there are also boom microphones which connect up via 2.5mm or 3.5mm just like the original microphone does. many of these are replacement microphones meant for other headsets so if you know of a certain headset with a removable microphone that you like the audio quality of, you can often swap to a different one as long as it has the same connection type (normally 3.5mm or 2.5mm and either straight on or requiring a 90 degree end). some you can find on the market quite easily to replace mics from turtle beach, astro and similar headsets or even unbranded gaming headsets. there are also a few universal models floating around as well which never had a headset. other times people have been known to buy a cheap generic headset and just take the microphone they want off it and use it with their headset.
lastly there are microphones which are meant to convert headphones with a removable cable into a microphone by clipping the mic into the headphone jack and replacing from the cable down with a replacement. this would be the boompro from vmoda though often it does not work well with many headphones and is more specific in what it fits and does not fit. that will not work in this case.
either option 1 or 2 would be best for you i'm thinking. you might have to do a bit of searching for option 2 but its entirely possible to find products that would just connect up. at first glance it looks like you would need one with a 90 degree 3.5mm end. the astro a30 or a40, mad catz tritton or ax, possibly turtle beach xo, and likely a few generic brand replacement headset mics are worth checking out the connections on or reviews to see if they will fit and/or be an upgrade to what you have now. of course, going with a modmic or high quality lavalier mic and just running a second cable down is likely going to be the best quality increase if you wanted to go that route.