My friend asked me to identify his plant. I'm just not sure about this one. It's not a cabomba... looks to me like Ambulia (Limnophila heterphylla) or hottoniflora, but cannot prove it.
Thanks.
It could also be Ceratophyllum demersum (Hornwort). It can get pretty thinned out if there's not enough light / CO2. I bought a bunch that looked just like that initially. Once it gets established it starts to get more 'bushy'.