Get yourself a small spray bottle and fill it with water. A mini water-gun can work too as it can fit easily in a pocket. Every time your friend swears give him/her a single squirt and say 'bad!'
Joking around aside, the first question is this; Does your friend actually swear that much, and are the words used actually that bad? Are you sure you aren't just reacting under your own ideals, trying to impose your perceptions? For example, is your friend swearing during inappropriate situations like in a professional setting?
Like at work/interviews, during formal gatherings, or other places/events that would normally call for a bit of impulse control?
Is your friend aware of when he/she swears? When a thing is done frequently, especially in speech, it becomes habitual and unconscious. It doesn't register as even happening and just comes out as a part of the flow of what is being said. Heck, many swears get used simply because they have actual grammatical functions. Or many, in the case of the 'F' word. Seriously, you can construct almost an entire sentence out of that one word and it would be grammatically correct. So it is easy to rely on them as a tool while speaking.
Third, does your friend consider his/her word choices to be a problem? Does your friend -want- to change the habit? And, looping back to the first question, is it something that even should change. Is it at risk of damaging your friend's wellbeing? IE; costing job interviews, offending customers/clients at work, making peers think they are being insulted or making them think he/she is being threatening? Or is this centered around a personal dislike on your part. ...In other words, is this something that your friend should change in order to improve, or do you just not like it.
If it is the latter, then some Self-reflection is in order as you may be the skewed individual. A bit of chill goes a long ways.
If this is genuinely a life-affecting issue (or potentially so), start by making your friend aware of what is happening. Without telling him/her what to do. For example converse about it as an observation.
"I've noticed lately that you swear an awful lot. I counted over the last few minutes we were talking, and it turns out you said 'x' word about 'x' times."
It might spark a conversation. For example if your friend says 'yes I know', that opens the door to ask why. If your friend says 'I didn't know', you can come up with a way to help your friend be aware of when he/she is swearing. Something friendly and non-invasive. Like holding up a finger for a second so your friend can see it, without actually interrupting the conversation. You can't change something you aren't consciously aware of doing. And likewise most often all it takes is gaining that awareness for a habit to change on its own.
Things to -really- avoid; commands or instructions, unless advice is first asked for it is the -fastest- way to shut a person down and set them to refuse your ideas. Right or wrong doesn't matter if a berson is put off, and no longer listening. Saying 'stop swearing', in any form or variation, will immediately kill a willingness to try.
Other options; Offer other words, adjectives, descriptors, etc that could replace the swears being used at the time. Broaden your friend's vocabulary. And in the process possibly your own too. Heck, make a game of it. Have pun wars, or rhyme wars; have a normal conversation, but as both of you talk make everything you say a pun, or a rhyme to what was said by the other person... respectively. It makes a person consciously aware of how they choose their words by creating a situation where talking takes active thought and ... almost planning.
Plus, it is fun as heck to see how long you can go without breaking the chain, and it can be hilariously creative too!
