Simply type in 'senior chat forums' into a search engine . Not all are good, this is by far the best... and it's important that she stays away from Fakebook , but a chat forum like this would be ideal for her.
How about you discuss it with her.. then if she's up for it, you sign her up, explain how it all works then introduce her ... and we'll take it from there.
She may now be so entrenched in being alone she'll find it hard to make the first move to make friends, so please help her get started here on this forum, show her how to register with a chosen name, how to post and how to navigate the forum of all it's topics. We have many subjects on here from General chit chat to..photography, ...to financial advice... to music forums where you can listen to or post your favourite music...to word games, ..all totally free, and once she's got her confidence up and later if she wants to go and enjoy other forums or game forums, she'll feel more confident in doing so...
She's in her 70's, that's really not old these days, but she will become very old very fast if she doesn't get her mind and body active again..... you sound very caring so do spend time with her just you and her or any other family members too, even if it's only sitting and chatting about something you know she enjoys...or take her out every so often, whether for a meal, or to the park or beach..or the store.. something so she doesn't feel no-one cares, and she's left with nothing but a box in the corner for a friend..