I began playing live tournament poker at the pool hall with a group that was dominantly elderly (60 and older).. I have built an immunity to go soft on them. They will take you for all you are worth!!
Seriously though, do not feel bad! This reminds me what my trainer told me after I lost my first fight. In my first MMA fight, I ended up losing in the 2nd round by a choke. I hardly threw a punch throughout the whole fight, and relied on my throws and takedowns, and went for submissions. At one point in the fight, I had my opponent in a full mount and could have ended it at the end of the 1st by pummelling my opponent for a TKO. I hesistated, because I would have felt bad for actually hurting him.
After the fight, my trainer gave me some tips, told me what I was doing right and what I was doing wrong, but most importantly, what he said was, "Don't worry about hurting your opponent. You both know what you're getting into, and he didn't hesistate to hit you, did he?"
What I'm trying to say is, 70 or not, that lady knew what she was getting into by sitting down at the table and risking her money. She would have done the same thing to you if she got the chance.