Question on Back stabber in the office

I started my current job one month ago and really love this job because I learn a lot from it and my boss & my boss's boos are very supportive on my development. I tried to maintain good relationship with everyone in the office because I am new and have less experiences than most of them.

However, I met a person I worked before in another company. He used to work as a consultant in a company where I was an intern. Unfortunately the manager in that company didn't want to keep more temporary staff here and let him go because he was not good as the other consultant. Furthermore my manager said that he was too "loud" and gossipy. He was not happy in his departure date and he didn't speak to me at all several weeks before his leave. Every time when I stepped into the office he will speak in a very low voice and others looked at me differently. I tried to speak with him several times, but found that his words became bitter, made me feel stupid before others. So I stayed away from him.

Unfortunately I met this guy again in the same company. Luckily we don't work in the same group and I become more experienced. I worked with a senior staff who sits near his cubicle. Yesterday afternoon when I tried to find the lady, I found she was off. Before I left this guy joked that she was hiding under the desk. So I joked back that why there are some nuts on the floor, any animals eat them. He was very sensitive, replied that he hates the nuts and give them to other people. Then he got a call from someone and asked me leave. I went away swiftly.

Today because of my work schedule I went to the senior staff's cubicle again. He sat there and murmured to her and both laughed loudly and weirdly on the other side. When he spotted me come, he suddenly stopped and mentioned something about the weather but with a strange smile on his face. He didn't leave when I spoken to the senior staff. So I told him that this is a private conversation and apologized that he had to leave. He was fumed and left.

When I backed to my cubicle I thought I can't let the conflict escalated before the senior staff (who will work with me with longer time in the future). I met him in the hallway and said that "I heard someone who used to work in XXX company mentioned that don't step on Brian's territory otherwise you should be careful" And "I am sorry" (His name is Brian.) He replied "whatever!" Obviously his face was green. The whole afternoon he laughed loudly and murmured in the office. Fortunately this didn't affect me because I had a lot of work to finish.

I know this is not over and expecting more. But I am concerned that in the future this will affect my work if the senior staff is on his side. And I know that he always buy her stuff to eat,lunches or chocolates.

Perhaps I also need to buy something for her, too. Any suggestions? Hope someone expert can help resolve this before it enlarges. Thanks a million!!!