Maintaining your professional reputation is important for career success and you should keep that in mind, as you go about performing your work every day. Writing a recommendation letter for someone is not the time to become 'complacent' about the image of professionalism you put forth for others to see. Complacency is a career killer! Recommendation letters should be clear and concise. You should stick to the facts about this employee's performance as an employee in your company. How well they met job expectations, did they work well with others? Did they have independent initiative that helped to get work done more efficiently? Did they have skills that were an asset to the company? Etc. A recommendation letter is not the time to 'get back' at someone just because your personalities never quite meshed.
Not unless this person's personality was so negative that it affected his/her productivity and the productivity of those working with them. In that case, it is far better to be up-front with the person asking for a recommendation letter from you. Honestly offer to them your feedback on their performance as an employee, and whether they really want you to write a recommendation letter that reflects those opinions. By the same token, you should avoid writing overly 'glossy' recommendation letters for those employees you have developed a close working relationship with, and especially with those you have developed friendships with outside of work. Again, stick to the facts about how this employee has performed in their capacity as an employee for your company.
Although it is "admirable" that your friend and co-worker picks up stray animals and finds them homes in their spare time-this has no bearing on how skilled they are at work. Not unless the work they are applying for is related to animal welfare. Always bear in mind that a recommendation letter is not just for the person for whom you write it. It also reflects upon you as a professional. If you write an overly "aggressive" recommendation letter, either with a positive or negative tone, this will 'diminish' your professional reputation among the other professionals. Don't insult the intelligence of those that a recommendation letter is meant to be forwarded to eventually.
Such actions will ultimately do more harm than good. Both to your professional reputation, but possibly to the reputation of the person for who you wrote the recommendation letter for.
John Murray publishes a blog filled with helpful articles and tips on writing great recommendation letters.