How to approach treading on someones toes

I have a client interested in what I do but she already has a company doing this for her. She's happy with them in so far as what they currently do but they aren't doing 1 particular thing that could make her site rank in a location.

Do I advise her to ask the present company to do these tasks and charge as an advisory role or shall I say I can take this on but how would I do that without pissing them off?

Maybe you have a way of dealing with this issue?

Who cares if you piss them off? They're the competition. If you can do it better, present your plan and sign her on. Businesses lose out to other businesses every day. It's all a part of the game - you either eat or you get eaten.