Where did you hear that?

Huh?
Don't take any further advice from the person that told you that.

They told me that bass is directional and that with all the open space, it has nothing to bounce off of. He said the best thing I could do in outdoor scenarios is to purchase woofers instead of subwoofers.

Pretty sure bass is not directional

@desmorider is correct - you are dealing with someone who has no idea what they are talking about.

If you are smart enough to post here, you should be smart enough to do some research on the Internet. Start here -
eaw.com

Sitting a woofer (or subwoofer, silly frequency dependent semantics) on the ground is exactly the same as Half Sphere Radiation.

+1

We can confirm your friend knows nothing of what he speaks....

There's no walls to reflect, and boost bass, so just give up? Forget that! The solution is bring more subs.

Actually, the answer is always Moar subs.

Yes

Depending on how much area you're attempting to cover, two QSC k12.2s or similar mains would be plenty. It's a graduation party, not a concert. Subwoofers would give you a fuller sound and more low end bass. I did a charity event Saturday and the area was a few acres, but a majority of the 200 or so people were in an area the size of a basketball court. The K.2s at 50% volume were perfect. I was playing a majority of R&B and Pop.

For 50-100 people subs are not really needed. Unless you want to get a workout in before and after the gig. Or if you have some crappy tops and/or small tops 10" or smaller.

Bring a sub

Inside with 50-100 people you can get away with no sub depending on size of room and type of speaker. Outside you should have the sub(s)

If you want the kids to feel the bass by all means use a sub. 1 is ideal for 50-100 kids.

Thanx for the responses guys! One more question....Coupled or not?

If they are identical, coupled is better.

I've used my sub for gigs outdoors and they give that extra oomph mate. Crowds loved it. Captures certain frequencies that can't be heard with regular speakers. Your biggest problem would be neighbors complaining (if you will be djing at night) bringing unwanted pigs in uniforms to shut ya down tbh

Do not confuse "good sound" with "loud sound". In fact, one of the real advantages of subs is you can have a deep, rich, full sound without having to crank the system.

It would also be a good idea to see if you can get a noise permit from the local city or town offices (it's usually free). Cops really don't like to get involved, but they have to when someone complains. You show them the noise permit, it gets the monkey off of their backs, everyone's happy except the complainer.