DDOS mitigation via pfsense

is it possible to mitigate a DDos attack from like say a botnet or something of that nature in order to prevent me from having it consume my data usage. apparently (huge emphasis on that word), someone i know was getting hit off so hard that it ate up his data plan on his Comcast subscription and had to pay for going over his limit. my question is if i were to make a pfsense firewall, would it be a good preventative measure to avoid said hypothetical scenarios like that from happening to me? 

I don't think that will work (could be wrong tho) because the firewall will be on your side of the isp modem so it will just be blocking the data through. Plus I think when you get DDos'ed they are pinging your ip so you'll have to find out if there's any blocking features on your modem.

Yeah, Once the traffic is at your firewall it's already been downloaded. The best way to stop it would be to change your IP or unplug your modem until they get bored. You could use snort in pfsense to detect and/or block a DOS, but I'm not sure if that would stop the traffic.

right, i guess my frustration is that people who can DOS can really mess up someones internet bill if they have a data cap. my friend said the attack he got affected his data usage. MY ISP on the other-hand (mediacom) said that a DOS attack would not add data to my bill. i guess im being a bit paranoid about this whole datacap scheme. 

Yeah, most ISPs will absolutely pitch a fit if you're getting a lot of DDoS traffic to your IP. Not only does it clog their network but its usually against their terms of service to do anything with servers, so yeah I'd be careful. They'll likely call you and complain if something does happen on a large enough scale to matter though.

but if im being attacked, doesn't the ISP have a legal obligation to go after the people who attack me?

If you are getting DDoS'd you should contact the ISP and tell them while it's happening. If nothing else, it's on record and you can fight the charges.