Another way around would be directly when saving the chages after putting the tag, it fills the description field with a code to display like :
<p style=”text-align:center; font-weight:bold;font-size:20px; color:#08a900″>Disponible
<span style=”font-weight:normal; font-size:12px;color:#000″>Pour plus d’informations, contactez-moi </span></p>
And if the field is not empty then it clears the field and fill it with the new code. That way when she will want to manage her site, she will just have to put the tag.