Can using ある in place of いる be offensive? Considering that is objectifying wouldn't it be offensive to use it in 's place when talking about a person, or would it just be seen as a mistake?

No, I don't think it would offend someone. You're probably concerned that it could offend someone because it sounds like you're treating someone as an object, not a person. Actually, many people think it is just a grammatical error, and they don't feel it is an offensive expression. They might take it as a mistake, or some people may not even understand as applying for a person almost never happens for native speakers. Some people might think you wanted to say (which means walk) because that is a common verb for a person.